When bidirectional communications are carried out between two voice telecommunications devices each having a microphone and a speaker, a sound (e.g., a sender's voice or background noise) collected by a microphone of one of the voice telecommunications devices is played back by a speaker of the other voice telecommunications device after being transmitted via an electrical communication network to the other voice telecommunications device on the other end of the communication. However, when the played-back sound goes around from the speaker directly or indirectly to the microphone and is collected, the played-back sound is returned to the voice telecommunications device which is the transmission source. Such a played-back sound returned to the voice telecommunications device which is the transmission source is called the “acoustic echo.”
As a device that suppresses an acoustic echo, an echo canceller which employs an adaptive filter is used widely. In this type of echo canceller, an adaptive filter successively estimates, i.e., learns the characteristics of a transmission path (also referred to as an “echo path” hereafter) via which an acoustic echo travels from a speaker to a microphone, and generates a false echo signal (also referred to as an “echo replica”). Then, by removing the false echo signal from the voice-transmission signal to be transmitted to the voice telecommunications device which is the transmission source, the acoustic echo component included in the voice-transmission signal can be reduced. Therefore, it can be said that the performance of the adaptive filter determines the echo cancellation performance of the echo canceller.
As a cause of an impediment to such the learning of the echo path characteristics which is carried out by the adaptive filter, there can be considered mixing of a disturbance signal into an acoustic signal detected by the microphone. For the adaptive filter, a near end speaker's voice or background noise which is mixed into an acoustic echo is a disturbance signal, and is an impediment to the learning of the echo path characteristics. Particularly, a double talk state in which a near end speaker's voice and an acoustic echo are mixed into the microphone becomes a problem as a cause of an impediment to the learning of the echo path characteristics.
As a measure against the above-mentioned problem resulting from a double talk, an echo canceller including an adaptive filter and a double talk detecting circuit for determining the presence or absence of occurrence of a double talk is known. This type of echo canceller is disclosed by, for example, Nonpatent Literature 1 mentioned below. An acoustic echo canceller disclosed by Nonpatent Literature 1 includes a double talk detecting circuit that monitors the amount of echo cancellation as a detection parameter, and that, when the amount of echo cancellation becomes equal to or larger than a predetermined amount, determines that a double talk has occurred and stops updating of an adaptive filter coefficient.
Further, an echo canceller that in order to improve the robustness against a disturbance signal of the learning of the echo path characteristics, includes two types of filters is also known. This type of echo canceller is disclosed by, for example, Nonpatent Literature 2 as listed below. The echo canceller disclosed by Nonpatent Literature 2 includes a background (BG) filter that learns the echo path characteristics, and a foreground (FG) filter in which a result of the learning carried out by the BG filter is incorporated over a certain period of time, and uses the FG filter for echo cancellation.
An echo canceller including two types of filters and a double talk detecting circuit is disclosed by Patent Literature 1 (Japanese Patent Application Publication No. 1994(H06)-338827).